Start FreeCAD.
Switch to FEM. This is important; otherwise the FEM preferences will not show up. Open Preferences, and switch to FEM->Calculix

Enter for ccx binary path the previously determined position. If you use the finder to select the binary, hit the keys CMD+SHIFT+. (the dot key).
Dont worry if the path changes after selection. It's only the symbolic link being expanded

Save the Preferences

Close FreeCAD. This is important, otherwise calculix might not be found.

After restarting FreeCAD, you can go through the example as described in the wiki.